When using multiple GPUs, is there a way to set Blender to use the memory from the card with the largest amount of RAM?
Blender is actually using the memory of BOTH cards. The scene has to be fully loaded into both GPUs as they are entirely separate. They cannot share memory. You are limited by the size of the smaller card. The order in the preferences makes no difference.

There is no way to fix this, and no way to have it not use selected cards with insufficient RAM to hold the scene. The only option is to set it to select only the larger card for the time being.